Valves of this type are preferably used by patients which have undergone stoma surgery. The excretion from the stoma should be allowed free passage to the reservoir of the bag, but must be prevented from passing backwards into the opening in the body of the patient, where it may cause an infection due to bacterial contamination of the contents of the bag, a deterioration of the general health of the patient resulting. As mentioned above known bags comprise a return valve in the shape of a flap of film resting against a film wall and with an opening facing downwards or two flags of film placed one on the other, said flaps also being provided with an opening face downwrads. Under normal conditions the foil flaps are able to ensure a total closure and besides they are simple and economical to produce, which makes it possible to discard the bag after a relatively short period of use in order to reduce the risk of contamination. However, the bag, which is being carried under the clothing of the patient, may under certain circumstances be bent or folded in such a way that the full flap is lifted from its abutment and the reservoir portion may incidentally be squeezed, its contents being pressed as a cascade against the return valve, which under such conditions has proved not to be as leakfree as under laboratory conditions.